This invention relates to the creation of Bragg reflective gratings in optical waveguides, typically optical fibre waveguides, by irradiation with ultra-violet light, through a mask, typically a phase mask, or alternatively by holographic means. One particular application for such reflective gratings is for chromatic dispersion equalisation in optical transmission systems. Such a use is described in the specification of U.S. Pat. No. 4,953,939.
Some applications for Bragg reflective gratings require a grating length which is longer than it is convenient to make a single mask for creating such a grating. There may, for instance, be a requirement to make a Bragg grating in the region of a metre long, whereas there are considerable difficulties in making an electron-beam fabricated mask much longer than about one hundred millimetres. A solution that has been proposed for overcoming this problem is to create the long grating step-wise in a succession of sections arranged end-to-end. Each section, except for the first to be created section, is created to commence at, or just beyond, the end of the next previously created section. If the long grating is designed for use in a wavelength division multiplexed (WDM) environment in which each section has a spectral bandwidth covering the whole spectrum of a single channel of the WDM signal, then it may be possible to arrange matters such that the reflection bands of the individual sections of the grating are spectrally separated by guard bands lying entirely within the spectral guard bands that separate the individual channels of the WDM signal. Under these circumstances any physical separation between adjacent sections of the grating is largely immaterial.
On the other hand, if breaks in the spectral reflection characteristic of the long grating are to be avoided, there is the problem that the spectral characteristic of one section will be cutting on at a point in the spectrum where the spectral characteristic of another section is cutting off. This means that both sections will be partially reflecting at a common wavelength. If the effective points of reflection are coincident, there is no problem. On the other hand, if one is longitudinally displaced from the other, then the two reflection components will coherently interfere, with the result that the magnitude of the resultant reflection is critically dependent upon the phase separation existing between the interfering components. A paper by R Kashyap et al entitled, `Super-step-chirped fibre Bragg gratings`, Electronics Letters (18 Jul. 1986) Vol. 32, No 15, pp 1394-6 explains that by deliberately arranging for adjacent sections of the grating not to abut, but to be separated by short intervening portions of waveguide, it is possible to make use of the photorefractive effect, and use UV light to trim the effective optical path length of any intervening portion to bring the phase separation of the two interfering components to a desired value. By this means it is possible to smooth out dips in the spectral reflection characteristic of the overall Bragg grating that can result from non-optimised intervals between adjacent sections of the grating. A drawback to this approach to lining up the sections is that it specifically requires a spacing between adjacent sections, and hence the delay time, the time taken by light of any particular wavelength to propagate from one end of the waveguide containing the Bragg grating to its point of reflection and back to the same end, is not a smoothly varying function of wavelength, but a function that contains as many steps, or more complicated discontinuities, as there are spaces between adjacent sections of grating, the delay in these discontinuous regions being affected by Fabry Perot type resonance effects between components of the same wavelength being reflected by the two adjacent grating sections.